IEnumerable not null, but throwing a NullReferenceException on iteration

Question

I have an IEnumerable that I run a foreach on. It's throwing a null reference exception in certain cases on the foreach line, it says

ienumerable threw an exception of type 'System.NullReferenceException

if (ienumerable != null)
{
    foreach (var item in ienumerable)
    {
        ......
    }
}

I put in a null check before the foreach loop, and the iEnumerable passes the null check, but then when I run the foreach loop on it it throws the null reference exception.

Answer 1

Iterators can do just about anything when iterated, including throw exceptions. So basically, you need to know what the source is. For example, this is a non-null iterator that throw in the same way:

var customers = new [] {
    new Customer { Name = "abc" },
    new Customer { },
    new Customer { Name = "def" }
};
IEnumerable<int> lengths = customers.Select(x => x.Name.Length);

this won't fail until the second time through the loop. So: look at where the iterator came from, and how the iterator is implemented.

Purely for fun, here's another that will fail identically:

IEnumerable<int> GetLengths() {
    yield return 3;
    throw new NullReferenceException();
}

Answer 2

the items in your ienumerable are sometimes null,

try this:

    if (ienumerable != null)
    {
        foreach (var item in ienumerable)
        {
            if(item != null)
            {
               // do stuff
            }

        }
    }

here is an example to try for you guys.

        string[] testStr = new string[] { null, "", "test" };
        foreach (var item in testStr)
        {
            if (item != null)
            {
                Console.WriteLine(item);
            }
            else
            {
                Console.WriteLine("item was null");
            }
        }

        Console.ReadKey();